datum

1. (dated) Singular of data: A single recorded material, especially obtained by scientific work.Examples: "Holonym: data set"Synonyms: data pointdatedform-ofsingular

2. (cartography, surveying) A point, line, or surface with reference to which positions (such as elevations) are measured or indicated (such as a permanent benchmark in leveling or mean sea level in a topographical survey).Examples: "A geodetic datum is sometimes also called a benchmark."; "In a strict sense, a tidal datum can be understood as the reference plane (or surface) to which the height of a predicted tide is referred.[…]Sounding and chart datums are low water datums, that is, they refer to the level of the water surface at low tide. Nonetheless, there are also datums based on high water levels.[…]These two different datums may be included in the broader category of vertical datums, which comprises any plane or surface used as a reference to measure vertical distances (such as depths, drying features, heights on shore, etc.)."; "Datums are another important map aspect related to projection. A datum provides a base reference for measuring locations on Earth's surface."

3. (nautical) A floating reference point, or SLDMB, used to evaluate surface currents in a body of water, and often employed by coastal search and rescue.